It took a few years to perfect the process, but building pedestrian timber bridges will be easier now with the introduction of new national standards that allow designers to plug in numbers and obtain reliable and structurally sound results—saving time and money while lessening the inconvenience to the public.
The new standards, created with the aid of a $40,000 grant from the USDA Forest Service, permit a designer, after capturing all relevant survey and geometric data for a job, to rapidly produce design drawings. This set of national standard design plans consists of 85 sheets of details and information covering numerous span length and width combinations, as well as timber species like red maple, southern pine and Douglas fir.
"These standards are a little like a cookbook approach to bridge design," said Patrick Powers, P.E. and president of Powers & Schram Inc., an engineering firm in State College, Pa., that developed the new national standards for pedestrian glulam timber bridges.
Typically, these timber bridges are made from local forest products using modern glued laminated technology. Glulam technology involves gluing together smaller pieces of timber to create larger structural members that have been shown through testing to be stronger and more durable than solid-sawn lumber.
"It’s been difficult to overcome the perception that timber bridges are too costly to construct," Powers said. "Standardization of the design process drastically changes that assumption because drawings that used to take weeks or even months to prepare using conventional design and drafting techniques take only days using the new standards."
The new standards follow LRFD specifications of the American Association of State Highway & Transportation Officials (AASHTO) and comply with the Americans with Disabilities Act.
Standardization of construction details for pedestrian timber bridges—like deck panel thickness, beam widths and hand rail member sizes—take the guesswork out of the process and result in significant manufacturing cost savings, according to Powers’ partner Perry Schram, P.E., as well as construction cost savings.
"As in any industry, significant cost savings can be realized from standardization," said Schram.
But the biggest savings in time, according to Schram, may actually be during the construction phase. A traditional concrete bridge deck must cure for up to 28 days before anything can travel over it, but once a glulam timber bridge deck is set, traffic can cross the same day.
"It’s tough to calculate the cost-savings in terms of time, but it’s there and it’s substantial," Schram said.
Plans allow flexibility
The new standard plans, which are currently available online from the USDA Forest Service and are available in an AutoCAD version from Powers & Schram Inc., incorporate designs and details for three different superstructure and substructure options.
The standards have already been used for a pedestrian bridge in Jacobsburg State Park in Wind Gap, Pa., in the eastern portion of the state. The single-span structure measures 39 ft in length and is 4 ft wide.
According to Powers, the Jacobsburg bridge is a perfect example of how glulam timber bridges can be an aesthetically pleasing alternative to traditional concrete structures that sometimes aren’t well-suited for recreational areas.
"Timber bridges are just a natural fit within state parks, municipal parks, recreation areas, golf courses, farms, university settings and private dwellings," Powers said. "When engineered correctly, timber bridges are strong and durable and can last 50 years or more, easily as long as traditional concrete and steel spans, and there is little maintenance with a timber structure.
"With these new standards, designing and constructing pedestrian timber bridges is quicker and easier, and in most cases the work can be done by a municipal maintenance crew, rather than a contractor," Powers said.
To obtain a copy of the new national standards, visit the USDA’s website (www.fs.fed.us/na/wit/newpubs.html).
To order full-size sheets and AutoCAD files for the new national standards, contact Powers & Schram Inc. at 814/238-1170 or send an e-mail request to [email protected].